Downhole in-situ heat generation to remove filtercake

ABSTRACT

In a method to degrade filtercake in a hydrocarbon reservoir using downhole in-situ heat generation, a filtercake breaker, configured to degrade filtercake at a temperature, is mixed with a drilling fluid. The drilling fluid mixed with the filtercake breaker is flowed through a hydrocarbon reservoir in which a wellbore is being drilled. A filtercake forms in a portion of the hydrocarbon reservoir responsive to flowing the drilling fluid. A temperature of the portion of the hydrocarbon reservoir is less than the temperature at which the filtercake breaker is configured to degrade filtercake. Multiple filtercake chemicals are flowed to the portion of the hydrocarbon reservoir. The multiple filtercake chemicals, when mixed, are configured to react in an exothermic reaction to release heat to increase the temperature of the portion of the hydrocarbon reservoir to at least the temperature at which the filtercake breaker is configured to degrade filtercake.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of and claims the benefit of priorityto U.S. patent application Ser. No. 14/965,242, filed Dec. 10, 2015, thecontents of which are incorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to generating heat in-situ in downholelocations, for example, hydrocarbon reservoirs.

BACKGROUND

Wellbores are drilled in hydrocarbon reservoirs to produce hydrocarbons(for example, oil, gas, or hydrocarbons or combinations of them) trappedin the hydrocarbon reservoirs. Drilling fluid is used to aid drillingwellbores in the hydrocarbon reservoirs. Drilling fluids (also known asdrilling muds) can be classified into three types—water-based muds,non-aqueous muds, gaseous drilling fluids. The drilling fluid servesmany roles, for example, providing a hydrostatic pressure to prevent thefluids in the formation from entering into the wellbore, keeping thedrill bit cool and clean during the drilling operation, carrying drillcuttings out of the wellbore and suspending the drill cuttings whendrilling is halted, to name a few.

During the drilling operation, reservoir drilling fluid is circulatedwithin the drilling equipment to cool the drill bit, reduce frictionbetween the drill string and the sides of the borehole and also to forma filtercake to prevent filtrate leak-off into the formation whendrilling through pay zone. The driving force for the formation of thefiltercake is the pressure applied to maintain the stability of theborehole. The filtercake, however, can restrict the flow of fluids intothe wellbore during the drilling process or during completion or both.Filtercake properties, for example, thickness, toughness, slickness,permeability, or other properties, can cause the pipe to stick or causeother drilling problems. Reservoir productivity can be compromised ifthe filtercake damage is not removed prior to or during completion ofthe well.

SUMMARY

This disclosure describes removal of filtercake from hydrocarbonreservoirs, for example, oil wells producing medium to heavy oil or oilwells found in low draw-down pressure reservoirs. This disclosure alsodescribes downhole in-situ heat generation, for example, using athermochemical reaction, to remove filtercake.

Certain aspects of the subject matter can be implemented as a method ofdegrading filtercake in a hydrocarbon reservoir. A filtercake breaker ismixed with a drilling fluid. The filtercake breaker is configured todegrade filtercake at a temperature. The drilling fluid mixed with thefiltercake breaker is flowed through a hydrocarbon reservoir in which awellbore is being drilled. A filtercake forms in a portion of thehydrocarbon reservoir responsive to flowing the drilling fluid. Atemperature of the portion of the hydrocarbon reservoir is less than thetemperature at which the filtercake breaker is configured to degradefiltercake. Multiple filtercake chemicals are flowed to the portion ofthe hydrocarbon reservoir. The multiple filtercake chemicals, whenmixed, are configured to react in an exothermic reaction to release heatto increase the temperature of the portion of the hydrocarbon reservoirto at least the temperature at which the filtercake breaker isconfigured to degrade filtercake.

This, and other aspects, can include one or more of the followingfeatures. The multiple filtercake chemicals can include a terpene basefluid and at least one of an organic acid or a catalyst solution. Theorganic acid can include dodecylbenzylsulfonic acid (DDBSA). Thecatalyst solution can include a Lewis acid. A byproduct of theexothermic reaction can exclude gases. To flow the multiple filtercakechemicals to the portion of the hydrocarbon reservoir, the multiplefiltercake chemicals can be mixed with the drilling fluid. The drillingfluid mixed with the multiple filtercake chemicals can be flowed to theportion of the hydrocarbon reservoir. The multiple filtercake chemicalscan be mixed with each other before flowing the multiple filtercakechemicals to the portion of the hydrocarbon reservoir. The multiplefiltercake chemicals can be mixed with each other at a surface of thehydrocarbon reservoir. To mix the multiple filtercake chemicals, a firstfiltercake chemical can be flowed from a surface of the hydrocarbonreservoir toward the portion of the hydrocarbon reservoir. After flowingthe first filtercake chemical, a second filtercake chemical can beflowed from the surface of the hydrocarbon reservoir toward the portionof the hydrocarbon reservoir. A difference in a density of the filterfiltercake chemical and a density of the second filtercake chemical cancause the first filtercake chemical and the second filtercake chemicalto mix before a mixture of the two reaches the portion of thehydrocarbon reservoir. A volumetric flow rate of at least one of thefirst filtercake chemical or the second filtercake chemical can becontrolled to cause the first filtercake chemical and the secondfiltercake chemical to mix before reaching the portion of thehydrocarbon reservoir. The filtercake breaker can be encapsulated beforemixing the filtercake breaker with the drilling fluid. Heat from theexothermic reaction can degrade an encapsulation surrounding thefiltercake breaker causing the filtercake breaker to directly contactthe filtercake.

Certain aspects of the subject matter described here can be implementedas a method of degrading filtercake in a hydrocarbon reservoir. Afiltercake breaker is encapsulated with an encapsulation configured toprevent the encapsulated filtercake breaker from directly contacting afiltercake. The filtercake breaker is configured to degrade filtercakeat a temperature. The encapsulated filtercake breaker is mixed with adrilling fluid. The drilling fluid mixed with the encapsulatedfiltercake breaker is flowed through a hydrocarbon reservoir in which awellbore is being drilled. A filtercake forms in a portion of thehydrocarbon reservoir responsive to flowing the drilling fluid. Theencapsulated filtercake breaker is embedded within the filtercake. Atemperature of the portion of the hydrocarbon reservoir is less than thetemperature at which the filtercake breaker is configured to degradefiltercake. Multiple filtercake chemicals are flowed to the portion ofthe hydrocarbon reservoir. The multiple filtercake chemicals, whenmixed, are configured to react in an exothermic reaction to release heatto increase the temperature of the portion of the hydrocarbon reservoirto at least a temperature at which the filtercake breaker is releasedfrom the encapsulation and the filtercake breaker is configured todegrade the filtercake in which the filtercake breaker is embedded.

This, and other aspects, can include one or more of the followingfeatures. The multiple filtercake chemicals can include a terpene basefluid and at least one of an organic acid or a catalyst solution. Theorganic acid can include dodecylbenzylsulfonic acid (DDBSA). Thecatalyst solution can include a Lewis acid. A byproduct of theexothermic reaction can exclude gases.

Certain aspects of the subject matter described here can be implementedas a method of degrading filtercake in a hydrocarbon reservoir. Afiltercake breaker is mixed with a drilling fluid. The filtercakebreaker is configured to degrade filtercake at a temperature. Thedrilling fluid mixed with the filtercake breaker is flowed through ahydrocarbon reservoir in which a wellbore is being drilled. A filtercakeforms in a portion of the hydrocarbon reservoir responsive to flowingthe drilling fluid. The filtercake breaker can be embedded within thefiltercake. A temperature of the portion of the hydrocarbon reservoir isless than the temperature at which the filtercake breaker is configuredto degrade filtercake. After the filtercake has formed, a terpenese basefluid is assed to the drilling fluid flowed to the portion of thehydrocarbon reservoir. After adding the terpene base fluid, an organicacid or a catalyst solution is added to the drilling fluid flowed to theportion of the hydrocarbon reservoir. The terpene base solution mixedwith the organic acid or the catalyst solution before reaching theportion of the hydrocarbon reservoir. When mixed, the terpene base fluidreacts with the organic acid or the catalyst solution in an exothermicreaction to release heat to increase the temperature of the portion ofthe hydrocarbon reservoir to at least the temperature at which thefiltercake breaker is configured to degrade filtercake.

This, and other aspects, can include one or more of the followingfeatures. A volumetric flow rate of at least one of the drilling fluidto which the terpene base fluid is added or the drilling fluid to whichthe organic acid or the catalyst solution is added can be controlled tocause the terpene base solution and the organic acid or the catalystsolution to mix before reaching the portion of the hydrocarbonreservoir. The filtercake breaker can be encapsulated before mixing thefiltercake breaker with the drilling fluid. Heat from the exothermicreaction can degrade an encapsulation surrounding the filtercake breakercausing the filtercake breaker to directly contact the filtercake. Abyproduct of the exothermic reaction can exclude gases.

The details of one or more implementations of the subject matterdescribed in this specification are set forth in the accompanyingdrawings and the description below. Other features, aspects, andadvantages of the subject matter will become apparent from thedescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a drilling operation in ahydrocarbon reservoir.

FIG. 2 is a flowchart of an example of a process for degradingfiltercake in a hydrocarbon reservoir.

FIG. 3 is a flowchart of an example process for degrading filtercake ina hydrocarbon reservoir.

FIG. 4 is a temperature versus time plot showing an increase intemperature resulting from mixing limonene with dodecylbenzylsulfonicacid (DDBSA).

FIG. 5 is a temperature versus time plot showing an increase intemperature resulting from mixing limonene with a DDBSA/Xylene blend.

FIG. 6 is a temperature versus time plot showing an increase intemperature resulting from mixing limonene and DDBSA with a 50%oil-based mud (OBM) contamination.

FIG. 7 is a flowchart of an example process for degrading filtercake ina hydrocarbon reservoir.

DETAILED DESCRIPTION

A filtercake breaker system is a chemical composition which, uponcontacting (for example, mixing, touching or otherwise contacting) afiltercake, reacts with the filtercake to degrade, deteriorate anddestroy the filtercake. Oil field breakers can be classified into threecategories: acids, enzymes and oxidizers. Acids are utilized to removefiltercake in payzones. Oxidative and enzyme breakers are used todegrade polymers in downhole applications. Oxidative breakers can beapplied in wider temperature range compared to enzyme-based breakersystems. Oxidative breakers are a class of chemicals that thermallydecompose and generate free radicals. Those radicals degrade themolecular weight of and reduce viscosity of polymers in a shortduration.

Delayed breaker systems are used in downhole applications to deliver thebreaker to a specific place in the wellbore. Certain breaker systems canbe activated by temperature, for example, the natural downholetemperature at the specific place to which the breaker is delivered, totrigger chemical release. In some situations, the downhole temperatureat the specific place at which the filtercake is formed may not besufficient to trigger delayed breaker release.

This disclosure describes a method to increase downhole temperature by athermochemical reaction, for example, an exothermic reaction. The heatemitted from the exothermic reaction acts as a trigger to activate adelayed breaker to release the breaker at the specific place in thewellbore. Such a delayed breaker system can be used to remove filtercakeat the specific place in the wellbore without needing to depend on thedownhole temperature at the specific place. Heat generated from theexothermic reaction can be utilized to control the rate of chemicalrelease from the delayed breaker.

FIG. 1 is a schematic representation of a drilling operation in ahydrocarbon reservoir. In FIG. 1, a well 11 has been or is being drilledthrough a hydrocarbon reservoir 10. The hydrocarbon reservoir 10 canspan across a subterranean zone (for example, a formation, a portion ofa formation or all or portions of multiple formations). The well 11 canextend across less than or all of one or more portions in thesubterranean zone. Production tubing 12 and a coiled tubing 13 extenddown into a wellbore 14 which extends into the hydrocarbon reservoir 10.During the drilling operation, a filtercake 15 is formed at the bottomof well casing 16 where the filtercake 15 meets the hydrocarbonreservoir 10.

FIG. 2 is a flowchart of an example of a process 200 for degradingfiltercake in a hydrocarbon reservoir, for example, the hydrocarbonreservoir 10. In some implementations, the process 200 can beimplemented by a drilling rig operator during the drilling operation orbefore completion of the wellbore, for example, the wellbore 14. At 202,a filtercake breaker is mixed with a drilling fluid, for example, anaqueous or non-aqueous drilling fluid. The filtercake breaker isconfigured to degrade or destroy filtercake at a temperature. That is,when a temperature of a portion of the hydrocarbon reservoir (orwellbore) in which the filtercake breaker is disposed reaches a certaintemperature, the filtercake breaker can degrade or destroy thefiltercake in the portion of the hydrocarbon reservoir (or wellbore). Aconcentration of the filtercake breaker in the drilling fluid can rangebetween about 0.01% to 2% by weight. In implementations in whichdelayed-release breaker system is used, a concentration of breakers inthe delayed-release breaker system can range between 80% and 50% byweight. OPTIFLO III™, which includes encapsulated ammonium persulfate,is an example of a delayed-released breaker system. Otherdelayed-release breaker systems can also be implemented.

Because the filtercake breaker is mixed with the drilling fluid, thefiltercake that is formed in any portion of the wellbore includes thefiltercake breaker embedded within the filtercake. That is, thefiltercake breaker will embedded in or be mixed with or be in contactwith the filtercake that will be formed in the wellbore at a later time.Consequently, the filtercake breaker need not be flowed to thefiltercake after the filtercake has formed in the wellbore. Rather, thefiltercake can be degraded or destroyed (or both) by triggering thefiltercake breaker which was embedded within the filtercake when thefiltercake was formed.

At 204, the drilling fluid mixed with the filtercake breaker is flowedthrough a hydrocarbon reservoir, for example, the hydrocarbon reservoir10, in which a wellbore, for example, the wellbore 14, is being drilled.A filtercake forms in a portion of the hydrocarbon reservoir (or thewellbore) responsive to flowing the drilling fluid. As described above,the filtercake breaker is mixed with or embedded within the filtercakethat forms in the portion. The temperature at which the embeddedfiltercake breaker triggers to degrade or destroy the filtercake can begreater than the temperature of the portion of the hydrocarbon reservoir(or the wellbore) in which the filtercake is formed. As described below,an exothermic reaction can be caused at the portion to increase thetemperature of the portion to at least the temperature at which thefiltercake breaker triggers to degrade or destroy the filtercake.

In some implementations, an embedding of the filtercake breaker withinthe filtercake can be controlled. For example, the filtercake breakercan be mixed with a drilling fluid prior to commencing the drillingoperation or during the early stages of the drilling operation. Doing socan cause the filtercake breaker to be mixed with the filtercakeregardless of where the filtercake is formed. Alternatively or inaddition, the filtercake breaker can be mixed with the drilling fluidwhen a particular portion of the hydrocarbon reservoir, for example, thezone in which the hydrocarbons are trapped, is being drilled. Doing socan cause the filtercake breaker to be mixed only with the filtercakeformed in the particular portion of the hydrocarbon reservoir. Inaddition, the mixing of the filtercake breaker can be stopped after theparticular portion of the hydrocarbon reservoir has been drilled. Doingso can cause subsequent filtercake to not include embedded filtercakebreakers.

At 206, multiple filtercake chemicals are flowed through the hydrocarbonreservoir to cause an exothermic reaction. For example, the multiplefiltercake chemicals are flowed to the portion of the hydrocarbonreservoir in which the filtercake forms. The multiple filtercakechemicals, when mixed, react in an exothermic reaction to release heatto increase the temperature of the portion of the hydrocarbon reservoirto at least the temperature at which the filtercake breaker isconfigured to degrade filtercake. In this manner, the filtercakechemicals react in an exothermic reaction to generate in-situ heat inthe wellbore at the portion of the hydrocarbon reservoir in which thefiltercake is formed. The generated heat increases the temperature ofthe downhole location to a temperature level at which the filtercakebreaker embedded within the filtercake is triggered. Once triggered, thefiltercake breaker reacts with the filtercake to degrade or destroy thefiltercake, thereby accomplishing filtercake removal.

FIG. 3 is a flowchart of an example process 300 for degrading filtercakein a hydrocarbon reservoir. In some implementations, at least some stepsof the process 300 can be implemented by a drilling rig operator duringthe drilling operation or before completion of the wellbore, forexample, the wellbore 14. At 302, a filtercake breaker is encapsulatedwith an encapsulation. The encapsulation, for example, a coatingsurrounding the filtercake breaker and containing the filtercake breakerwithin the coating, prevents the encapsulated filtercake from directlycontacting a filtercake. Coated or encapsulated filtercake breakers canbe implemented to achieve controllable filtercake breaker release ratesin downhole applications or to release filtercake breakers at specificlocations in the hydrocarbon reservoir (or the wellbore) or both. Suchencapsulated filtercake breakers can be triggered based on downholeconditions, for example, temperature, pH or other downhole conditions,to trigger filtercake breaker release through the encapsulation.

At 304, the encapsulated filtercake breaker is mixed with a drillingfluid similar to step 202 described above with reference to FIG. 2. At306, the drilling fluid mixed with the filtercake breaker is flowedthrough a hydrocarbon reservoir, for example, the hydrocarbon reservoir10, in which a wellbore, for example, the wellbore 14, is being drilled,similar to step 204 described above with reference to FIG. 2. Afiltercake forms in a portion of the hydrocarbon reservoir (or thewellbore) responsive to flowing the drilling fluid. As described above,the encapsulated filtercake breaker is mixed with or embedded within thefiltercake that forms in the portion. Because the encapsulation withinwhich the filtercake breaker is contained prevents the filtercakebreaker from directly contacting the filtercake, the filtercake breakerdoes not degrade or destroy the filtercake. Sometimes, in addition, thetemperature at which the embedded filtercake breaker triggers to degradeor destroy the filtercake can be greater than the temperature of theportion of the hydrocarbon reservoir (or the wellbore) in which thefiltercake is formed. As described below, an exothermic reaction can becaused at the portion to increase the temperature of the portion to atleast the temperature at which the encapsulation releases the filtercakebreaker and, if necessary, the filtercake breaker triggers to degrade ordestroy the filtercake.

At 308, multiple filtercake chemicals are flowed through the hydrocarbonreservoir to cause an exothermic reaction. For example, the multiplefiltercake chemicals are flowed to the portion of the hydrocarbonreservoir in which the filtercake forms. The multiple filtercakechemicals, when mixed, react in an exothermic reaction to release heatto increase the temperature of the portion of the hydrocarbon reservoir.The increased temperature can be sufficient to break the encapsulationto release the filtercake breaker to directly contact the filtercake.Such direct contact can cause the filtercake breaker to degrade ordestroy the filtercake. In addition to breaking the encapsulation, theincreased temperature can also be sufficient to trigger the filtercakebreaker to degrade filtercake. In this manner, the filtercake chemicalsreact in an exothermic reaction to generate in-situ heat in the wellboreat the portion of the hydrocarbon reservoir in which the filtercake isformed. The generated heat increases the temperature of the downholelocation to a temperature level at which the filtercake breaker embeddedwithin the filtercake is released from the encapsulation and, ifnecessary, triggered. Once released or triggered, the filtercake breakerreacts with the filtercake to degrade or destroy the filtercake, therebyaccomplishing filtercake removal.

In any of the implementations described in this disclosure, the multiplefiltercake chemicals can include a terpene base fluid and at least oneof an organic acid or a catalyst solution. The mixture of the terpenebase fluid with the organic acid initiates the polymerization of theterpene fluid (for example, d-limonene, pinene or other terpene fluid)which is highly exothermic as described below. The exothermic reactioncan cause a temperature differential sometimes in excess of 300° F. Thebyproduct of the exothermic reaction excludes gases. In other words, theexothermic reaction resulting from using the multiple filtercakechemicals described here generates only exotherms (i.e., heat) and doesnot generate any gaseous byproducts. In some instances, the byproductscan include a low molecular weight resin material, but no gases. Becausegases are not generated as a byproduct of the exothermic reaction, thepressure at the location will not be affected by the reaction. Moreover,a rate of the exothermic reaction described here is high allowingdownhole temperature to increase rapidly. Thus, the exothermic reactionbe implemented for high volumetric flow rates of drilling fluids. Also,higher reaction rates enables better controlling a time of the chemicalrelease from delayed breakers.

In some implementations, the terpene base fluid can include limonene andthe organic acid can include dodecylbenzylsulfonic acid (DDBSA). FIG. 4is a temperature versus time plot showing an increase in temperatureresulting from mixing limonene with DDBSA. As shown in FIG. 4, theaddition of DDBSA causes a slow exotherm about 5 seconds after additionfollowed by a rapid temperature increase from about 100° F. to about400° F. In some implementations, the terpene base fluid can includelimonene and the organic acid can include a DDBSA/Xylene blend. FIG. 5is a temperature versus time plot showing an increase in temperatureresulting from mixing limonene with a DDBSA/Xylene blend. As shown inFIG. 5, the addition of about 12 milliliters (ml) of DDBSA/Xylene blendto about 20 ml of limonene resulted in a 250° F. temperaturedifferential. In some implementations, the terpene base fluid caninclude limonene and the organic acid can include DDBSA with a 50%oil-based mud (OBM) contamination. FIG. 6 is a temperature versus timeplot showing an increase in temperature resulting from mixing limoneneand DDBSA with a 50% OBM contamination. As shown in FIG. 6, mixing about20 ml of limonene, about 10 ml of DDBSA and about 20 ml of oil-based mudcauses an exothermic reaction with an upper exotherm limit beingcomparatively lower than upper lower therm limits in the plots shown inFIGS. 4 and 5, respectively.

In any of the implementations described in this disclosure, the multiplefiltercake chemicals can be flowed to the portion of the hydrocarbonreservoir (or wellbore) in which the filtercake is formed byimplementing the following techniques. The multiple filtercake chemicalscan be mixed with the drilling fluid, and the drilling fluid mixed withthe multiple filtercake chemicals can be flowed to the portion of thehydrocarbon reservoir (or wellbore). In some implementations, themultiple filtercake chemicals can be mixed with each other beforeflowing the multiple filtercake chemicals to the portion of thehydrocarbon reservoir. For example, the multiple filtercake chemicalscan be mixed with each other at a surface of the hydrocarbon reservoir,for example, the surface 20 of the hydrocarbon reservoir 10. In suchimplementations, the filtercake chemicals can be pumped in the sametubing through which the drilling fluid is pumped.

In some implementations, the filtercake chemicals can be pumped usingdedicated tubing separate from the drilling fluid tubing. The length ofthe dedicated tubing can be selected based on a location of thefiltercake in the hydrocarbon reservoir (or wellbore). Using dedicatedpumps, the filtercake chemicals can be pumped from the surface 20 of thehydrocarbon reservoir 10 through the dedicated tubing. In suchimplementations, the filtercake chemicals can be pumped in a carrierfluid, for example, the drilling fluid. Pumping the filtercake chemicalsusing dedicated tubing and pumps can allow flowing the filtercakechemicals to the portion of the hydrocarbon reservoir (or wellbore) at aflow rate (or flow rates) that is (or are) different from the flow rateof the drilling fluid during drilling operation.

In some implementations, to mix the multiple filtercake chemicals, afirst filtercake chemical, for example, one of the terpene base fluid orthe organic acid or the catalyst solution, can be flowed from a surfaceof the hydrocarbon reservoir toward the portion of the hydrocarbonreservoir by adding the first filtercake chemical to the drilling fluid.After adding the first filtercake chemical, a second filtercakechemical, for example, the other of the terpene base fluid or theorganic acid or the catalyst solution, can be flowed from the surface ofthe hydrocarbon reservoir toward the portion of the hydrocarbonreservoir by adding the second filtercake chemical to the drillingfluid. A different in a density of the first filtercake chemical and adensity of the second filtercake chemical causes the two filtercakechemicals to mix before a mixture of the two filtercake chemicalsreaches the portion of the hydrocarbon reservoir. The in-situ heatgenerated from the exothermic reaction that results from the mixturereleases or triggers the filtercake breaker (or both) to degrade ordestroy the filtercake. In some implementations, a volumetric flow rateof at least one of the multiple filtercake chemicals can be controlledto cause the first filtercake chemical and the second filtercakechemical to mix before reaching the portion of the hydrocarbonreservoir.

FIG. 7 is a flowchart of an example process 700 for degrading filtercakein a hydrocarbon reservoir. In some implementations, the process 700 canbe implemented by a drilling rig operator during the drilling operationor before completion of the wellbore, for example, the wellbore 14. At702, a filtercake breaker is mixed with a drilling fluid similar to step202 described above with reference to FIG. 2. The filtercake breaker mayor may not be encapsulated prior to mixing with the drilling fluid. At704, the drilling fluid mixed with the filtercake breaker is flowedthrough a hydrocarbon reservoir, for example, the hydrocarbon reservoir10, in which a wellbore, for example, the wellbore 14, is being drilledsimilar to step 204 described above with reference to FIG. 2. Afiltercake forms in a portion of the hydrocarbon reservoir (or thewellbore) responsive to flowing the drilling fluid. As described above,the filtercake breaker (encapsulated or unencapsulated) is mixed with orembedded within the filtercake that forms in the portion. At 706, aterpene base fluid is added to the drilling fluid after the filtercakehas formed. At 708, after adding the terpene base fluid, an organic acidor a catalyst solution is added to the drilling fluid. In someimplementations, the terpene base fluid can be pumped in a pill followedby a thin barrier fluid. The organic acid, for example, DDBSA, orcatalyst solution, for example, a catalyst solution containing Lewisacid, can then be pumped downhole. The resulting exothermic reaction cangenerate a temperature differential that is sufficient to increase adownhole temperature to a temperature at which the filtercake breaker istriggered (or released from the encapsulation and then triggered) todegrade or destroy the filtercake.

The exothermic reactions described above were caused by mixing twofiltercake chemicals. In some implementations, more than two filtercakechemicals can be mixed to cause the exothermic reaction or to adjust atemperature differential resulting from the exothermic reaction. In someimplementations, the filtercake chemicals used to cause the downholeexothermic reaction can include monomers such as acrylates, acrylamides,styren, or other monomers with initiators such as acid, caustic, radicalgenerating Azo and peroxide species. Other examples of filtercakechemicals can include combinations of polyamines and fatty acids mixeddownhole resulting in initial salt formation with a temperature inexcess of 70° C. In general, any combination of filtercake chemicals canbe used to cause the exothermic reaction (for example, organic acids andbases, inorganic acids and bases such as Alkali metal hydroxides andhydrochloric acid) as long as the byproducts of the reaction excludesgases.

Thus, particular implementations of the subject matter have beendescribed. Other implementations are within the scope of the followingclaims.

The invention claimed is:
 1. A method of degrading filtercake in ahydrocarbon reservoir, the method comprising: encapsulating a filtercakebreaker with an encapsulation configured to prevent the encapsulatedfiltercake breaker from directly contacting a filtercake, wherein thefiltercake breaker is configured to degrade filtercake at a temperature;mixing the encapsulated filtercake breaker with a drilling fluid;flowing the drilling fluid mixed with the encapsulated filtercakebreaker through a hydrocarbon reservoir in which a wellbore is beingdrilled, wherein a filtercake forms in a portion of the hydrocarbonreservoir responsive to flowing the drilling fluid, wherein theencapsulated filtercake breaker is embedded within the filtercake,wherein a temperature of the portion of the hydrocarbon reservoir isless than the temperature at which the filtercake breaker is configuredto degrade filtercake; and flowing a plurality of filtercake chemicalsto the portion of the hydrocarbon reservoir, wherein the plurality offiltercake chemicals, when mixed, are configured to react in anexothermic reaction to release heat to increase the temperature of theportion of the hydrocarbon reservoir to at least a temperature at whichthe filtercake breaker is released from the encapsulation and thefiltercake breaker is configured to degrade the filtercake in which thefiltercake breaker is embedded, wherein a byproduct of the exothermicreaction excludes gases, wherein a pressure at the portion of thehydrocarbon reservoir is unaffected by the exothermic reaction.
 2. Themethod of claim 1, wherein the plurality of filtercake chemicalscomprises a terpene base fluid and at least one of an organic acid or acatalyst solution.
 3. The method of claim 2, wherein the organic acidcomprises dodecylbenzylsulfonic acid (DDBSA).
 4. The method of claim 2,wherein the catalyst solution comprises a Lewis acid.
 5. The method ofclaim 1, further comprising mixing the plurality of filtercake chemicalswith each other before flowing the plurality of filter cake chemicals tothe portion of the hydrocarbon reservoir.
 6. The method of claim 5,wherein mixing the plurality of filtercake chemicals with each othercomprises mixing the plurality of filtercake chemicals with each otherat a surface of the hydrocarbon reservoir.
 7. The method of claim 1,wherein mixing the plurality of filtercake chemicals comprises: flowinga first filtercake chemical of the plurality of filtercake chemicalsfrom a surface of the hydrocarbon reservoir toward the portion of thehydrocarbon reservoir; and after flowing the first filtercake chemical,flowing a second filtercake chemical of the plurality of filtercakechemicals from the surface of the hydrocarbon reservoir toward theportion of the hydrocarbon reservoir, wherein a difference in a densityof the first filtercake chemical and a density of the second filtercakechemical causes the first filtercake chemical and the second filtercakechemical to mix before a mixture of the first filtercake chemical andthe second filtercake chemical reaches the portion of the hydrocarbonreservoir.
 8. The method of claim 7, further comprising controlling avolumetric flow rate of at least one of the first filtercake chemical orthe second filtercake chemical to cause the first filtercake chemicaland the second filtercake chemical to mix before reaching the portion ofthe hydrocarbon reservoir.